The present invention relates to an exhaust gas component purifying catalytic material, and an exhaust gas component purifying catalyst which comprises a catalyst support substrate having thereon a catalyst layer containing the exhaust gas component purifying catalytic material.
In vehicles, such as automobiles, an exhaust gas component purifying catalyst is provided in an exhaust gas passage of an engine to purify exhaust gas discharged from the engine. Particularly, an exhaust gas component purifying catalyst for a vehicle equipped with a diesel engine or a lean-burn gasoline engine is required to have a function of minimizing unburned carbon particulates (so-called “particulate matter (PM)”) and nitrogen oxides (NOx) contained in exhaust gas.
For instance, on concern that unburned carbon particulates (so-called “particulate matter (PM)”) contained in exhaust gas of a diesel engine lead to environmental pollution, particulate matter emissions allowances are regulated by law. For complying with the regulations, a diesel particulate filter (DPF) is provided in an exhaust gas passage of a diesel engine-mounted vehicle. The DPF is made of a heat-resistant ceramic material, such as silicon carbide (SiC) or cordierite, and formed in a given structure, called “three-dimensional mesh type” or “wall flow type”, so that PM in exhaust gas is trapped in the DPF during the course of passing therethrough.
Under engine operating conditions, trapped PM will be gradually accumulated in the DPF along with an operating time to cause an increase in exhaust back pressure, leading to problems, such as a decrease in engine power and in fuel economy. As measures therefor, there has been known a technique of supplying an excessive amount of HC component (e.g., fuel) from an upstream side of an exhaust gas passage, and burning the HC component by a catalyst disposed upstream of the DPF to increase a temperature of the DPF based on heat from the burning, so as to promote light-off and burning of the PM. In late years, it has also been proposed to provide a catalyst layer containing an oxygen-absorbing material together with alumina and others, on a surface of an exhaust-gas path in the DPF, so as to effectively promote the light-off and burning of the PM through the use of an oxygen component absorbed and stored in the oxygen-absorbing material.
The oxygen-absorbing material has a property capable of absorbing and storing oxygen in exhaust gas when an exhaust atmosphere is in an oxygen-excess state, and releasing the oxygen stored therein when the exhaust atmosphere is in an oxygen-deficient state (so-called “oxygen absorbing/releasing capability”) Based on this property, an air/fuel ratio (A/F) range allowing for purification of HC, CO and NOx is expanded to provide is enhanced exhaust gas purification performance (i.e., enhanced exhaust gas conversion performance).
Heretofore, a composite oxide containing Ce and Zr has been known as the oxygen-absorbing material. Concerning this type of composite oxide, for example, in JP2004-174490A, the applicant of this application has proposed an oxygen-absorbing material having a catalytic metal (exactly, catalytic noble metal) arranged at a lattice position or an interlattice position of a crystal structure thereof, and an exhaust gas purifying catalyst comprising this oxygen-absorbing material. Specifically, the JP 2004-174490A discloses a technique of arranging a catalytic noble metal at a lattice position or an interlattice position of a crystal structure of a composite oxide containing Ce and Zr. This technique makes it possible to provide enhanced oxygen absorbing/releasing capability while preventing sintering of the catalytic noble metal and thereby maintain a high conversion performance over a long period of time. The JP2004-174490A also discloses an oxygen-absorbing material containing neodymium (Nd) in addition to the Ce, Zr and a catalytic noble metal.
The applicant has also proposed a catalyst in a catalyst layer formed on the wall surface of an exhaust gas flow path of DPF comprising a catalytic noble metal for oxidizing/burning PM, and a composite oxide free of Ce, wherein the composite oxide contains Zr and a rare-earth metal Y (e.g., yttrium), to act as an oxygen ion-conducting material (see, for example, JP2005-262184A). Differently from the aforementioned oxygen-absorbing material, when a difference in oxygen-concentration occurs around grains of the oxygen ion-conducting material in the catalyst, oxygen ions are transferred from a first region having a relatively high oxygen concentration to a second region having a relatively low oxygen concentration through the grains, and active oxygen is released from the second region. This makes it possible to facilitate expanding a burnable region after generation of a flame kernel for oxidizing/burning PM, so as to efficiently oxidize/burn the PM.
On concern that NOx contained in exhaust gas of a diesel engine or a lean-burn gasoline engine designed to perform combustion in an oxygen-excess state lead to environmental pollution, NOx emissions allowances are also regulated by law. As one of the measures for complying with the regulations, there has been a technique of providing a NOx absorbing/reducing catalyst in an exhaust gas passage of a diesel engine or a lean-burn gasoline engine to purify NOx contained in exhaust gas, in such a manner as to absorb NOx contained in exhaust gas, in the NOx absorbing/reducing catalyst, during lean exhaust gas conditions in a normal engine operation, and temporarily create a rich exhaust gas condition to reduce the NOx absorbed in the NOx absorbing/reducing catalyst.
The NOx absorbing/reducing catalyst involves a problem about deterioration in catalytic performance due to a reaction with sulfur components contained in fuel. As one example of a measure against this problem, JP 2006-043533A discloses a technique of catalytically reducing nitrogen oxides even in the presence of sulfur oxides, using a catalyst which comprises an oxide catalytic component consisting of: ceria, or praseodymium oxide, or a mixed oxide or composite oxide of at least two elements selected from the group consisting of cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum.
In cases where a particulate filter is provided in a vehicle equipped with a diesel engine or a lean-burn gasoline engine, measures are often taken to promote the burning of PM in such a manner that an oxidation catalyst for oxidizing HC and others is disposed upstream of the particulate filter, and a given amount of fuel controllably increased by so-called “post injection” (i.e., an additional injection subsequent to first injection(s), called “pilot injection”) is burned in the oxidation catalyst to increase a temperature of exhaust gas to be introduced into the particulate filter. In this case, an extra amount of fuel is inevitably consumed by the oxidation/burning of the PM. Thus, there remains the need for developing a composite oxide capable of achieving a higher PM burning speed than that in the composite oxide containing Zr and a rare-earth metal Y, as disclosed in the JP 2005-262184A, so as to minimize an increase of the fuel. Further, as an amount of PM to be burned increases, an amount of carbon monoxide (CO) emission is likely to undesirably increase due to occurrence of incomplete burning of the PM. Thus, it is necessary to take measures against the incomplete burning of the PM.
In addition to the need for minimizing PM, a vehicle equipped with a diesel engine or a lean-burn gasoline engine is expected to further enhance NOx conversion performance. Although the JP 2006-043533A discloses a catalyst adapted to catalytically reduce nitrogen oxides, there remains a need for developing a composite oxide capable of achieving further enhanced conversion performance of NOx in exhaust gas.